Photophysical Studies of Helicate and Mesocate Double-Stranded Dinuclear Ru(II) Complexes.

The metal-ligand charge transfer ( 3 MLCT) and phosphorescence-quenching metal-centered ( 3 MC) states of the helicate and mesocate diastereoisomers of a double-stranded dinuclear polypyridylruthenium(II) complex have been investigated using ultrafast transient absorption spectroscopy. At 294 K, transient signals of the helicate decayed significantly slower than those of the mesocate, whereas at 77 K, no clear contrast in kinetics was observed. Contributions to excited-state decay from high-lying 3 MLCT states were identified at both temperatures. Spectroscopic data (294 K) suggest that the 3 MC state of the helicate lies above the 3 MLCT and that the reverse is true for the mesocate; this was further validated by density functional theory calculations. The stabilization of the 3 MC state relative to the 3 MLCT state in the mesocate was explained by a reduction in ligand field strength due to distortion near the ligand bridge, which causes further deviation from octahedral geometry compared to the helicate. This work illustrates how minor structural differences can significantly influence excited state dynamics.